1. Field of the Invention
The present invention relates to a slip-ring arrangement for electrical machines.
2. Description of the Prior Art
An arrangement of this general type is known from German Auslegeschrift No. 1,488,655. The transfer of current from brushes to slip rings is known to involve losses, specifically in the form of frictional and current-passage losses, the quantity of heat arising having to be eliminated by means of a ventilation system governed by the amount of losses which occur. In the conventional cooling arrangement for slip rings, an air flow is guided along the brush surface and, if appropriate, also through axial and/or radial channels in the slip rings.
In slip-ring arrangements for turbo-generators, as are known, for example, from Swiss Patent Specification No. 443,468, three radial fans are provided between two slip rings, the middle fan sucking fresh air in through axial bores in both slip rings and the outer fans each sucking fresh air in along the surface of one slip ring, all three conveying the heated air outwardly.
Swiss Patent Specification No. 410,152 relates to a slip-ring arrangement with a device for cooling the slip rings by means of a coolant blown through channels in the slip rings, and in this arrangement slots or grooves present in the slip-ring surface intersect with parallel-axis channels in the slip-ring body, the gaseous coolant being fed to these channels via channels extending approximately radially. In a further development of this known slip-ring arrangement, German Offenlegungsschrift No. 2,426,823 proposes forcing the coolant flow through the channels by means of a radial fan located between the slip rings.
In slip-ring arrangements for asynchronous machines, in which there are, as a rule, three slip rings located comparatively close to one another, the cooling concepts known in turbo-generators cannot be implemented in practice. In most cases, the space existing between the individual slip rings is too slight to accomodate fans and appropriate coolant-conveying devices. There has, therefore, been no shortage of attempts in the past to solve the problem of cooling several slip rings located next to one another.
Thus, German Patent Specification No. 504,351 describes an arrangement for cooling the slip rings and for removing the brush dust in which a slip-ring support designed as a hollow body is connected at one axial end to an air suction hose and is provided on its periphery with air inlet orifices located between the slip rings. Special air-guide attachments are fitted to the slip-ring support outside the air inlet orifices for the purpose of directional air guidance. The suction hose is connected to a vacuum line. This known solution is expensive in terms of construction, leads to a substantial enlargement of the slip-ring diameter and has, in particular, the disadvantage that it does not permit direct cooling of the contact surface of the slip ring.
In a slip-ring arrangement known from German Auslegeschrift No. 1,488,655, for electrical machines with several slip-rings located next to one another, such slip rings are worked into these channels and are provided with inlet and outlet orifices for supplying and discharging a gaseous coolant. The slip rings are made hollow, and the interior is connected via the inlet orifices at several points distributed along the periphery, to insulated tubes which each lead to the end face of the outermost slip ring. In this case, the outlet orifices are formed by oblique bores leading radially outwardly in the end walls of the slip ring. Alternatively, passages can also lead from the interior of the slip rings to the slip-ring surface, in which case, according to the particulars of this Auslegeschrift No. 1,488,655, the brushes have to be arranged so that they do not cover these outlet orifices.
Although the slip-ring arrangement permits directional flow of coolant to the individual slip rings, nevertheless it essentially possesses two disadvantages. On the one hand, the cost in terms of construction is increased considerably as a result of the design of the slip rings as hollow bodies, and, on the other hand, the coolant flow described causes serious fouling of the brush space, which can impair freedom of movement of the brushes in their holders and also jeopardize the insulation, that is to say, such considerably reduces the tracking resistance. Furthermore, the supply or discharge of coolant, which is presently desirable, is, if possible, guided by the contact surface of the brushes and thus expressly allows the heat to be eliminated directly at its point of origin.